What does this have to say when painting ? First of all this means that you can't simply pick a white color for highlights, a middle skin color and black as shadow color and have Painter to calculate the in-between colors to a gradient.
Try to use the entire scale and stretch the lightning further into the shadow.Some famous and talented artist don't use black (in its pure form) at all. There is always some light in darkness.

Two Gradients and Photoshops Gradient Editor with a 3-color custom gradient

In the image above I have used Photoshop to generate the left gradient using the Gradient Editor to build a custom 3-color gradient. I picked a light skin color, a middle skin color and black and left Photoshop to calculate the in-between colors. The gradient on the right is made from more than 15 colors.
The difference is that the light and dark colors are more saturated in the right gradient than in the left. I think the left gradient is mixed with too much gray, and it does not give the saturated, red shadows areas you often see on skin. Though, again this isn't a rule you can follow blindly.